1. Field of the Invention
The present invention relates to an improvement of a speaker in which distortion of reproduced sound is eliminated by a filler.
2. Description of the Related Art
It has been general for a cone speaker, a dome speaker or a horn speaker that damping is performed with electromagnetic force occurring in a voice coil mainly to improve a transient characteristic of a speaker.
With respect to the damping based on the electromagnetic force of the voice coil as described above, when a speaker is used while connected to an amplifier through a network or an attenuator, there is a disadvantage that damping force is reduced in a fixed frequency region and thus damping runs short, so that the transient characteristic is deteriorated.
That is, a network divides audio current output from an amplifier into respective reproduction bands suitable for speakers such as a tweeter, a squawker, etc Attenuation of the audio current which is divided into the respective frequency bands of the reproduction bands is normally equal to −3 db at cross-over frequencies, however, it increases to −3 db or more at the outside of an in-use band. Therefore, large resistance (impedance) caused by the attenuation of the audio current (electrical input) is interposed between the speaker and the amplifier at the outside of the in-use band. Therefore, the damping based on the electromagnetic force does not work, and thus the damping force runs short at the outside of the in-use band, so that the transient characteristic is also degraded.
Furthermore, a method of attaching foamed rubber or the like to the back surface of a vibrating plate or putting a flexible sound absorbing material in a space at the back surface of the vibrating plate while the flexible sound absorbing material is brought into contact with the vibrating plate, thereby making these materials function as damping resistors, has been adopted as a method for damping high-band divisional vibration of a hard dome and vibration of a piston with which the vibrating plate moves integrally. However, this method has not been so effective to improve the frequency characteristic and the transient characteristic.
In order to solve the above problem, the present inventor has disclosed a speaker in which an acoustic resistor formed of a soft porous material is provided between a vibrating plate and a holder provided in proximity to and substantially in parallel to the vibrating plate or between an equalizer and a vibrating plate in the following patent document 1.
With respect to the speaker concerned, it has been possible to suppress degradation of the transient characteristic caused by lack of damping force of the speaker, particularly lack of electromagnetic force occurring at the outside of an in-use band of a speaker used in a network, and also acoustic damping force to the piston vibration with which the vibrating plate moves integrally is generated, thereby greatly improving the frequency characteristic and the transient characteristic.
However, in the speaker described above, the acoustic resistor formed of the soft porous material is provided between the vibrating plate and the holder or the equalizer, and thus the distance between the vibrating plate and the holder or the like must be made remarkably small in order to obtain effective damping force. Therefore, when the displacement of the vibrating plate increases, the soft porous material is pressed and increased in density, and thus the acoustic resistance value varies due to variation of the thickness of the soft porous material. Accordingly, there is a problem that distortion occurs in reproduced sound.
Therefore, the present inventor has disclosed a speaker in which the acoustic resistor is improved to have a multilayered structure made of a soft porous material and a hard porous material in the following patent document 2.
With respect to the acoustic resistor having the multilayered structure as described above, even when the soft porous material is compressed and thus increased in density in the case of large displacement of the vibrating plate, air flows through the hard porous material, so that substantially fixed acoustic resistance force is obtained at all times irrespective of magnitude of the displacement of the vibrating plate, the porous materials well follow vibration of the vibrating plate and thus the frequency characteristic and the transient characteristic are improved.
Afterwards, the present inventor has repetitively made studies on the improved speaker described in the patent document 2, and as a result he/she has found that distortion occurs in reproduced sound during operation and it has an adverse effect on the frequency characteristic, etc.
That is, in the speaker described in the patent document 2, the soft porous material and the hard porous material are stacked in the layered form, and thus air flows excellently through the hard porous material at the peripheral portion. However, air hardly flows at the center portion, and thus distortion is liable to occur in reproduced sound.    Patent Document 1: JP-B-03-059639    Patent Document 2: JP-A-01-320900